EVM is a measure of the distortion present on the signal, and it is often degraded by linearity limitations in the PA. For TDD technologies, achieving good EVM can be challenging because the characteristics of the signal path must stabilize quickly at the beginning of each transmit burst.

In terms of modulation, as noted above, both technologies use OFDMA for downlink. In the uplink, LTE technology uses SC-FDMA to reduce the relatively high PAPR found in OFDMA. SC-FDMA shares several characteristics in common with OFDMA. For example, they are both block-based modulation schemes where channel equalization is performed in the frequency domain. In some ways, SC-FDMA could be regarded as “DFT-precoded (discrete Fourier transform)” or “DFT-spread” OFDMA.

While both of these modulation technologies divide the transmission bandwidth into smaller subcarriers, a major difference between them is how they use data symbols within these carriers. OFDMA uses a number of narrowband subcarriers, extending for the entire duration of the symbol (Fig. 1). SC-FDMA has a shorter symbol time, but it occupies a wider bandwidth per data symbol.¹

Figure 2 shows the effects of this lower PAPR on EVM, where the measured PAPR for SC-FDMA is about 2 dB lower than for OFDMA. This means that, for the same sized PA, an LTE system can transmit approximately 2 dB more power than a WiMAX PA, meeting a –28-dB EVM specification.

However, there are other considerations. For instance, the OFDMA system will likely perform better in fading environments and can use receivers that are less complex. And while SC-FDMA reduces PAPR, which can result in more efficient PAs, the duplexer losses from using FDD in LTE will likely negate any gains from SC-FDMA. As a result, the two systems will be approximately equal from a transmit point of view.

While the idea of a converged PA architecture for WiMAX and LTE is still only a possibility, technologies are in place to make it a likely path. Multiband PAs are already in production for WLAN and cellular, so it is possible to develop them to cover the fragmented spectrum for WiMAX and LTE.

BiCMOS PAs are in development, and they can enable adaptive bias control, which will drive higher efficiency for PAs, as will digital predistortion and analog linearization techniques. In addition, biCMOS can be used for a variety of other control functions, making it possible to include a serial interface for features such as multiband PA selection, tunable filters, and switch control with fewer control lines.

Combined, these existing technologies lay the groundwork for true WiMAX/LTE coexistence in a single PA. When that occurs, the opportunity for wireless voice, video, and data ubiquity may be truly in our grasp.

Reference

1. Agilent Technologies, “3GPP Long Term Evolution: System Overview, Product Development, and Test Challenges,” http://cp.literature.agilent.com/litweb/pdf/5989-8139EN.pdf, 2008.

John Brewer Jr. is vice president, Corporate and Business Development, with SiGe Semiconductor. He holds a bachelor of science degree in electrical engineering from Santa Clara University.

Peter L. Gammel is the chief technical officer and vice president of engineering at SiGe Semiconductor. He holds a PhD in physics from Cornell University and bachelor of science degrees in physics and mathematics from the Massachusetts Institute of Technology.

Darcy Poulin is a principal systems engineer at SiGe Semicondcutor. He is a WiMAX Forum Certified RF Engineer. Also, he holds a bachelor of science degree with honors in engineering physics from Queen’s University at Kingston and a PhD in applied physics from McMaster University in Hamilton, Ontario.

Related Articles

1. If You Want To Save The Environment, Start By Recycling Your Cell Phone
2. NTIA Unveils National Broadband Map